RAF kinases have been associated with cancer since their discovery in 1983. The serine/threonine protein kinase BRAF (B-Raf) is an important player in the receptor tyrosine kinase (RTK)-mediated mitogen-activated protein kinase (MAPK) pathway, where it is activated by the RAS small GTPase. BRAF not only activates the MAPK pathway that affects cell growth, proliferation, and differentiation but also affects other key cellular processes, such as cell migration (through RHO small GTPases), apoptosis (through the regulation of BCL-2), and survival (through the HIPPO pathway). The BRAF gene was found constitutively activated by mutation in 15% of all human known cancer types, in particular in melanoma (Davies et al., 2002, Nature, 417(6892):949-954). BRAF was reported to be mutated at several sites; however, the vast majority of mutated BRAF are V600E (1799T>A nucleotide change), characterizing up to 80% of all BRAF mutations (Davies et al., 2002, supra). This mutation results in amino acid change that confers constitutive kinase activity. Most of the BRAF mutations result either in the acquirement of new phosphomimetic residues or in the release of the auto-inhibitory conformation imposed by the N-terminal region, which enhances the dimerization of the kinase domain, a crucial process for kinase activity. According to recent studies, BRAF is reported to be mutated in about 8% or all cancers and approximately one half of all melanomas harbor a BRAFT1799A transversion which encores the constitutively active BRAF-V600E oncoprotein (Holderfield et al., 2014, Nature Reviews, 14, 455-467). The BRAF V600E mutation is also the most common genetic mutation detected in patients with papillary thyroid cancer (PTC) and occurs in approximately 45% of patients (Davies et al., 2002, supra). Mutations in the BRAF gene are also found in about 10% of colorectal carcinoma (CRC) patients, 6% in lung cancers and nearly all cases of papillary craniopharyngioma, classical hairy-cell leukaemia (HCL-C) and metanephric kidney adenoma (Holderfield et al., 2014, supra) are usually associated with significant poorer prognosis (Tejpar et al., 2010, Oncologist, 15(4):390-404).
BRAF inhibitors have been developed by different companies and the most commonly used are vemurafenib (marketed as Zelboraf by Roche) and dabrafenib (marketed as Tafinlar by GSK), but others exist such as LGX818 (encorafenib; Novartis), XL281 (Exelixis), and CEP-32496 (Ambit Biosciences Corporation) (Zhang, 2015, Curr Opin Pharmacol., 23:68-73). Impressive clinical results have shown that targeted therapies for melanoma, i.e. BRAF and MEK inhibitors, can efficiently treat highly mutagenic solid malignancies by blocking critical cell survival pathways. However, subsequent clinical observations have demonstrated that these inhibitors, even when combined, are rarely curative and that resistance almost inevitably develops within a few weeks or months (Tannock et al., 2016, N Engl J Med, 375:1289-94). Currently, great efforts are being deployed to understand the mechanisms behind this resistance (Smalley, 2010, J Invest Dermatol, 130:28-37), and to develop other combinatory therapies to target the compensatory pathways that become activated in response to pathway inhibition, e.g. the PI(3)K (Phosphatidylinositol-4,5-bisphosphate 3-kinase) pathway (Jokinen et al., 2015, Ther Adv Med Oncol, 7: 170-80). Nevertheless, the likelihood of resistance to these new combinations is also high. The somatic evolution of malignant tissues involves dynamic emergent processes including nondeterministic alterations of genetic pathways (Gatenby et al., 2002, Cancer Res, 62:3675-84) and non-genetic adaptive responses of malignant cells to their environment and to therapies which affect intra-tumor heterogeneity (Tannock et al., 2016, supra).
Targeted therapy-induced mutations, the resulting genetic heterogeneity and the consequent induced resistance are irreversible processes. Irreversible genetic resistance contrasts with the reversible non-genetic adaptive resistance to these drugs documented in many recent studies (Nazarian et al., 2010, Nature, 468:973-7; Sun et al., 2014, Nature, 508:118-22; Boussemart et al., 2014, Nature, 513:105-9), which occurs in vivo in an undetermined proportion of malignant cells, qualified here as reversible tumor heterogeneity (RTH).
Therefore, tumor plasticity and the heterogeneous response of melanoma cells to targeted therapies are major limits for the long term efficacy of this line of therapy. If BRAF mutated melanoma develop resistance during the course of the BRAF inhibitor therapies, the response of other BRAF-driven cancers to BRAF inhibitors is even more disappointing: only ˜5% of BRAF-mutant colorectal patients respond to vemurafinib (Kopetz et al., 2010, J Clin Oncol., suppl: 28), and thyroid cancers do not respond to selumetinib (Hayes et al., 2012, Clin Cancer Res., 18:2056-65).
Targeting tumor plasticity is theoretically possible through the modulation of the expression of RNA-binding proteins which can affect many different and compensatory mechanisms of the adaptive response of malignant cells to targeted therapies within the cancer signaling network (Cui et al., 2007, Mol Syst Biol, 3:152).
Human antigen R (HuR) is a modulator of gene expression and a trans-acting factor in the mRNA-processing machinery used in the cell stress response which is mainly considered as a tumorigenic protein, partly because some of its targets are cell cycle- and apoptosis-regulating proteins and partly because its expression pattern increases in some malignancies (Abdelmohsen et al., 2010, Wiley Interdiscip Rev RNA, 1:214-29). However, HuR involvement in cancer is highly complex (Cho et al., 2012, J Biol Chem, 287:14535-44) according to the available in vivo experimental data (Yiakouvaki et al., 2012, J Clin Invest, 122:48-61; Giammanco et al., 2014, Cancer Res, 74:5322-35). Based on various studies HuR, is estimated to have between 5′000 to more than 7′000 direct mRNA targets (Lebedeva et al., 2011, Mol Cell, 43:340-52) and within the cancer signaling network (Cui et al., 2007, supra), at least 10% of the nodes, including highly connected ones, belong to the HuR repertoire. Agents capable of modulating nucleo-cytoplasmic expression of the protein HuR/ELAV have been developed, for example for the treatment of metabolic disorders (Roche et al., 2014, Cellular Signalling, 26, 433-443) and their effect on the modulation of expression of HuR/ELAV has been described as accessory and not considered as being crucial for their current therapeutic use.
Undoubtedly, there is a need to develop effective treatments for BRAF mutated solid tumor cancers, in particular melanoma that would prevent the development of drug resistance.